SPT Testing in Madison: Accurate Blow Counts for Glacial Soils

We have seen projects on Madison's isthmus face costly delays when designers assumed uniform soil conditions based on a few scattered boreholes. The reality underfoot, shaped by the Wisconsin glaciation, is a complex layering of till, outwash sands, and lacustrine silts that can change N-values dramatically over short distances. A generic geotechnical assumption simply does not hold here. Running a systematic SPT investigation following ASTM D1586 lets you map these transitions early, so foundation elements are sized for the actual stratigraphy rather than a best guess. In our experience, pairing SPT data with a triaxial shear test clarifies strength parameters for the clay-rich tills that appear near Lake Mendota, while a quick grain size analysis helps distinguish clean outwash sands from silty zones that could complicate drainage design.

The value of an SPT in Madison is not the N-value alone, but the sequence of N-values that reveals how the glacier deposited each layer.

Our service areas

Our approach and scope

The rig most suited for Madison's terrain is a truck-mounted CME-75 or equivalent, equipped with an automatic trip hammer calibrated to deliver 140-pound blows over a 30-inch drop. The auto-hammer provides consistency that reduces operator variability, which is critical when penetrating the dense basal till found across much of Dane County. The split-spoon sampler is driven 18 inches in three 6-inch increments, with the blow count for the final 12 inches recorded as the N-value. When the sampler hits a cobble embedded in the glacial drift—a common occurrence near the terminal moraine south of town—the driller logs refusal and the engineer notes the potential for oversized particles that a standard sampler cannot capture. In these cases, supplementing the SPT with a CPT sounding can provide a continuous profile through the problematic interval without the need to recover a disturbed sample.

SPT Testing in Madison: Accurate Blow Counts for Glacial Soils — Technical reference — Madison

Local geotechnical context

A mid-rise structure on East Washington Avenue encountered a classic Madison problem: the upper 15 feet were stiff glacial till with N-values above 30, but a 6-foot seam of soft, normally consolidated silt lay hidden between 18 and 24 feet. The initial SPT program, which terminated at 20 feet on every other boring, missed the weak layer entirely. The design team specified shallow spread footings based on the upper till, only to discover differential settlement cracks two years after occupancy. A deeper SPT grid, extending to at least 30 feet across all boreholes, would have caught the compressible silt. The cost of the extra drilling was negligible compared to the underpinning work that followed. Madison's glacial stratigraphy is simply too variable to truncate explorations at a single arbitrary depth.

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Explanatory video

Reference standards

ASTM D1586 - Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils, ASTM D2487 - Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), IBC 2021 - International Building Code, Chapter 18: Soils and Foundations, ASCE 7-22 - Minimum Design Loads and Associated Criteria for Buildings and Other Structures

Reference parameters

Parameter	Typical value
Hammer type	Automatic trip hammer, ASTM D1586 compliant
Hammer weight	140 lb (63.5 kg)
Drop height	30 inches (760 mm)
Sampler	Standard split-spoon, 2-inch OD, 1.375-inch ID
Drive increments	Three 6-inch intervals, N-value from last 12 inches
Borehole diameter	4 to 8 inches, rotary wash or hollow-stem auger
Typical depth range in Madison	30 to 60 feet below grade

Frequently asked questions

How much does a typical SPT boring cost in Madison?

For a standard SPT boring on an accessible site within the Madison metro area, the cost generally runs between US$550 and US$700 per boring, assuming a depth of 30 to 40 feet in typical glacial soils. The final figure depends on total footage, number of borings, and whether the site requires track-mounted equipment for soft ground or limited-access setups.

How deep should SPT borings go for a commercial building in Madison?

The IBC requires borings to extend to a depth where the stress increase from the foundation is less than 10 percent of the existing effective overburden stress. In Madison's glacial deposits, this typically means 30 to 60 feet for a multi-story commercial structure. We often go deeper when the site is near the Yahara River or along the isthmus, where softer lacustrine sediments can persist at depth.

What corrections do you apply to the raw SPT N-values?

We apply the standard corrections per ASTM D1586 and the NCEER guidelines: energy correction for the auto-hammer efficiency, rod length correction for depths below 10 feet, sampler correction for liners or lack thereof, borehole diameter correction, and overburden stress correction to obtain the normalized N1₆₀. Each corrected value is reported alongside the raw blow count so the design team can trace every adjustment.

Location and service area

We serve projects in Madison and surrounding areas.

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